Generally, an air compressor to be used for a pneumatic tool converts the rotary motion of an electric motor into the reciprocating motion of a piston inside a cylinder via a crankshaft, compresses air sucked in from a suction valve by the reciprocating motion of the piston, and stores the compressed air in an air tank part from an exhaust valve of the cylinder, as disclosed in Patent Document 1 mentioned below. This air compressor is carried together with a pneumatic tool into a work site, such as a construction site. Then, the air compressor is used as a drive source to drive a nail or screw into a work member like a lumber by supplying the compressed air stored in the air tank part of the air compressor to a pneumatic tool (e.g., nailing machine) via an air hose.
As this air compressor is carried together with a pneumatic tool into an indoor or outdoor work site and supplies compressed air in the air tank part to the pneumatic tool via an air hose, the air compressor is generally of a portable type which has a relatively small-sized air tank. This portable air compressor has a relatively smaller capability of generating compressed air to be stored in the air tank as compared with a floor type air compressor, and is demanded of having as small an air tank as possible for excellent portability.
In the conventional air compressor, as disclosed in Patent Document 1, air sucked into the cylinder is compressed by the reciprocating motion of the piston, generating compressed air. The piston is driven by converting the rotary motion of the electric motor into reciprocating motion. Then, the air compressor stores higher compressed air into the air tank as the revolving speed of the electric motor is set higher by a control circuit which controls the rotary motion of the electric motor. In this case, a pressure sensor for converting compressed air into a voltage signal is installed in the air tank, and the control circuit acquires pressure (P) in the tank part from a detection signal from the pressure sensor.
When the pressure sensor detects that the pressure (P) in the tank part reaches a maximum set pressure value set as the upper limit for the purpose of safety, the control circuit stops operating the electric motor. If compressed air having an air pressure equal to or higher than the use limit pressure of a pneumatic tool is stored in the air tank, even when the pneumatic tool to be connected demands a larger amount of compressed air than can be produced by the generation capacity of the air compressor, it is possible to cope with the demand by discharging the compressed air in the air tank for a predetermined time.
When the pressure in the air tank drops to or below a predetermined restart set pressure value due to consumption of compressed air in the air tank, on the other hand, the control circuit restarts the electric motor to generate compressed air and store it in the air tank. Further, the control circuit detects a drop (ΔP) of air pressure per a predetermined passed time (ΔT) based on the detection signal from the pressure sensor to acquire a pressure drop rate (ΔP/ΔT) of the air in the tank. Then, the control circuit determines if the air consumed by the work with the pneumatic tool is large or small, and sets again the revolving speed of the electric motor and the set value of the restart pressure corresponding to the pressure drop rate (ΔP/ΔT). In this manner, the control circuit performs control to keep the air pressure in the air tank at the pressure which can be used by a pneumatic tool, thus ensuring efficient use of the pneumatic tool.
For example, in a pressure change curve diagram based on the control operation of the conventional air compressor as shown in FIG. 6B, an initial restart set pressure value (second restart set pressure value) is 3.2 MPa. In case of a large amount of air consumption, at the time of working, i.e., when the pressure drop rate (ΔP/ΔT) is large, a first restart set pressure value for generating compressed air is set to a high value, e.g., 4.0 MPa. Then, at a time point c where the pressure in the air tank drops to or below 4.0 MPa, the electric motor is operated at a relatively high revolving speed of, for example, 2600 rpm to start storing compressed air in the air tank early to cope with the large amount of air consumption. This secures the time of usage of the pneumatic tool till a time point d where the pressure drops to or below the capacity limit pressure (forced operation set pressure) of the air compressor.
In case of a small amount of air consumption in the air tank, i.e., when the pressure drop rate (ΔP/ΔT) is small, the set value of the restart pressure is set to 3.2 MPa smaller than the set value of 4.0 MPa. Until a time point h where the air pressure drops from the set value of 4.0 MPa to reach 3.2 MPa, the air compressor is not restarted and stands by. At the time point h where the air pressure drops to or below 3.2 MPa, the control circuit performs control to set the revolving speed N of the motor to a low revolving speed N3=1600 rpm to execute an operation of restoring compressed air.
In this manner, the control circuit operates the electric motor and the air compression part with the restart set pressure value and the revolving speed of the motor changed according to the size of pressure drop rate (ΔP/ΔT) of the amount of air consumption in the air tank. This can eliminate wasteful operations of the electric motor part and the piston part, thus making it possible to reduce wasteful power consumption and prevent wear-off or failure of the air compressor.
As an air compressor which is controlled by another conventional control system, there is known an air compressor which is configured to have a changeover switch capable of setting the revolving speed of the electric motor to one of a high revolving speed and a low revolving speed, regardless of the amount of air consumption in the air tank, so that a user of a pneumatic tool selects the changeover switch beforehand to set the operational condition.
[Patent Document 1] Unexamined Japanese Patent Application KOKAI Publication No. 2004-300996